He converted to Islam before the conquest of Mecca and immigrated to the Prophet(P) [i.e. in Medina]. he embraced Islam after the Hijrah and joined the Muslims in Medina. Thus, his apostasy occurred later which means it occurred in Medina.
Concerning Chapter 6 (from which the verse 6:93 is quoted), many reports support the fact that it was entirely revealed in Mecca. They also go on saying that this Chapter was escorted by 70,000 angels when Gabriel carried it down to the Prophet.
In a nutshell, Ibn Abî Sarh embraced Islam after the Muslims had immigrated to Medina. He took the trouble to migrate to Medina where he became one of the scribes of the Prophet. For an unknown reason, he apostatized and went back to Mecca. He is supposed to have told the Meccans that he changed the Qur'ân according to his own will. This seems to be very predictable for someone in his situation seeking the favours of the Meccans whom he betrayed not a long time before. Abdullâh was among the bunch that had to be executed but he could benefit of Uthmân's intercession and he kept his life safe. Though the beginning of his Islam was unstable (he migrated then apostatized then converted back to Islam in a very short time), he became a good Muslim and was even made the commander of Muslim troops.
Full article:
http://www.islamic-awareness.org/Quran/Sources/Sarh/
